Generally, with respect to low-contrast laser-printed characters and stamp or seal characters, it is difficult to obtain sharp binary images because of blurs or stains on the background thereof. To solve the problem, Applicant proposes a method of automatically deciding an optimum binarizing level corresponding to a change in background level. This method is an improvement over the conventional method in which the binarizing level is set to a fixed value (reference is made, for example, to Japanese Patent Application No. 158032/1985.)
A binarizing apparatus in accordance with the proposed method comprises a converter for converting a pixel density level into a digital quantity, a two-dimensional local memory for storing a value of density of each pixel to form a move mask of a predetermined size, a detection circuit for detecting the minimum (or maximum) value of density in the move mask, and a correction circuit for correcting the representative value of the background level represented by the minimum (or maximum) value in the move mask.
In the method, the variation in background level depending on the position is detected by a move mask, and a threshold for binarizing of a target pixel is decided by reference to a representative background level in the move mask with addition of a predetermined correction value to thereby perform binarizing appropriately for every position. If the move mask is formed to have an area of N.times.N meshes (Pixels) sufficiently larger than the line width in the line graphic, it is possible to make any one of the mesh points in the move mask be a background point.
In this case, however, if binarizing is made with a thresh-old level near the density value of the background, the blurs or stains in the background are also binarized, thereby making it difficult to read the characters, though the lines of the characters are reproduced thickly. Conversely, if binarizing is made with such a threshold level as not to reproduce blurs or stains in the background, character lines to be reproduced become blurred. In short, because lines are not discriminated from blurs or stains, the conventional binarizing apparatus binarizes the density values of the same degree equally.